Although skin cancer can develop on any part of the body, it's most commonly found on areas that are often exposed to the sun. The National Cancer Institute (NCI) and ACS say there are different types of skin cancer:

* SkinBasal cell cancer, the most common, is relatively slow-growing in many cases. It begins in the lowest layer of the epidermis, called the basal cell layer. It can first appear as a waxy bump or a flat lesion. * Squamous cell cancer is also slow-growing but it can metastasize or spread to other body parts. It begins in the middle portion of the epidermis. This type of cancer can also start as a pearly bump or a flat lesion. It may be crusted as well. * Malignant melanoma is the most dangerous type. It originates in the melanocytes, or pigment producing cells of the skin. It starts as a pearly or dark bump or as a small lesion with irregular borders. About a third of the cases start in moles that suddenly change appearance or start to bleed. * Other nonmelanoma skin cancers account for less than one percent of nonmelanoma skin cancers, according to ACS. These less common skin cancers include: o Kaposi's sarcoma, which usually starts within the dermis but can also form in internal organs. The tumors consist of bluish-red or purple lesions. This cancer occurs in people with compromised immune systems, such as those with HIV infection or AIDS, as well as transplant recipients who are on immune-suppressing drugs. o cutaneous lymphoma, a type of lymphoma that begins in the skin. o skin adnexal tumors, rare tumors that start in the hair follicles or sweat glands, and are usually benign. Skin adnexal tumors can be found on skin throughout the body, including the eyelids. o sarcomas, which usually start in tissues deep beneath the skin, but can develop in the skin as well. o Merkel cell carcinoma, a rare cancer that develops on or just beneath the skin and in hair follicles, and usually appears as firm, shiny skin lumps. It is very aggressive, so early detection and treatment are essential.

There is another condition, actinic keratosis (AK), which appears as rough, red or brown scaly patches on the skin. AK is known as a precancerous condition because it sometimes develops into cancer. Like skin cancer, it usually appears on sun-exposed areas but can be found elsewhere. The National Institutes of Health say AK's may start as small, red, flat spots then grow larger and become scaly or thick, like a wart. Most AKs form on the face or back of the hands, but can appear elsewhere.

When examining your skin for changes or growths, the American Academy of Dermatology says there are "ABCD" warning signs to be aware of:

* Asymmetry - one half of a growth does not match the other half. * Border irregularity - the edges are ragged, notched or blurred. * Color - the pigmentation is mottled or not uniform. * Diameter - the width is greater than six millimeters (about the size of a pencil eraser). * Evolving - a mole or growth that looks different or is changing in its size, shape, or color

In general, any unusual change in the skin, especially a new growth or sore that doesn't heal, can be a warning sign of skin cancer, and should be checked promptly by your physician. Any growth of a mole should be of concern too. When caught early, your chances for successful treatment are much higher.